1. Field of the Invention
The present invention concerns a method for determining an EMG-signal out of a raw signal obtained via a number of electrodes that interact with a patient to detect signals from the patient's diaphragm, the signals being supplied from the electrodes via respective signal channels.
2. Description of the Prior Art
Sensing of the EMG-activity in a patient's diaphragm by placing a catheter with a number of electrodes down the esophagus is a known technique, which is described in, among others, U.S. Pat. No. 5,671,752. The EMG-signals, which are receiving by handling the signals, can be used in connection with mechanical ventilation of patients, which is described, among others, U.S. Pat. No. 5,820,560 and PCT Application WO 98/48877.
Sensing of the EMG-activity from the diaphragm can even be done outside the body by placing electrodes on the patient, described, for example, in U.S. Pat. No. 4,248,240.
The physiological activity in the diaphragm generates a relatively weak electrical signal. This is so in particular if it is compared with the considerably stronger physiological (and electrical) activity in the heart (EKIG-activity). Therefore, the present desire is to attain in the best way the highest quality possible for the signal handling of the raw signal which the sensors detect, so that the resulting EMG-signal in the highest degree possible corresponds to the physiological activity. This is evident even in PCT Application WO 01/03579. In PCT Application WO 01/03579 it is assumed that the electrodes location in relation to the center of the diaphragm is known. Then the electrodes are measures based on location and symmetry, in which the EKG signal is measured in a traditional way.
Previously known methods for compensating for the EKG signal include, among others, using a band-pass filter which filters out the frequencies where the EKG signal normally appears. It is also known to measure the EKG signal separately and then remove an equivalent signal from the measured EMG signal.
A specific problem which is present with interference from EKG-activity is that the frequency spectrum for the physiological EKG-activity partially overlaps the frequency spectrum for the physiological EMG-activity. Moreover, there is the fact that the EKG-disturbances appear at different time points in relation to the breathing cycle.
None of the above-described methods consider the actual disturbance the EKG signal creates in a particular measuring situation in a particular patient. This disturbance also varies with time.
The known techniques also fail to consider that the respective frequency spectrum for EMG and EKG can vary between measurements in different people.